There is an increasingly dependence on wireless devices. Upcoming wireless technologies are only going to increase the dependency. In the coming years, not only will cell-phones and laptops be wireless, but also cars, houses, appliances such as coffee machines, and even our body organs and almost anything may be wireless enabled. The Internet of everything (IoE) is gasping to seamlessly network us with the physical world.
The expansion of wireless use is expected to fundamentally change the way we live, work and play. However, various engineering challenges need to be tackled before this opportunity could be exploited. The future wireless devices should handle a deregulated and much more energetic spectrum. In addition, to service diverse applications scenarios, wireless devices will be desired to be software-defined and omni-functional. That is, the user should be able to significantly alter the functionality of his/her device at a click of a button, without interfering with other wireless operations.
A Software-defined radio (SDR) implements most of its functionalities using software running on a generic hardware (Mitola, Software radios-survey, critical evaluation and future directions, 1993). Because it is simpler to design, implement and maintain, SDR is supposedly cheaper compared to a hardware extensive radio. Furthermore, SDR allows radio devices to be used in a manner similar to that of personal computers where the user buys generalized hardware and installs the appropriate software to obtain the operation desired. Such scale of flexibility inspires end-user's innovation, which is vital to maintain the growth of the wireless industry in the 21st century.
When a SDR is capable to autonomously select its parameters, it is called cognitive radio (CR) (Mitola, Cognitive Radio: An Integrated Agent Architecture for Software Defined Radio, 2000). A CR uses artificial intelligence (AI) to dynamically control its parameters. Currently, CR is primarily being applied in dynamic spectrum access (DSA) based wireless communication systems. The DSA technology allows a secondary user to use a licensed band of the spectrum when not being used by its primary user. Compared to the current radio spectrum management, a DSA based policy requires multiple radios to share frequency bands together. This is a significant shift in spectrum management as it means radios may act as direct interferers to each other. CRs incorporate flexibility and artificial intelligence in the radio design. This allows us to share the spectrum efficiently and dynamically even when interferers exist in the designated frequency band.
In other words, SDR and CR technologies have benefits that the wireless industry cannot afford to lose. Nevertheless, SDR and CR are not widely adopted because of the difficulties involved in the design of their analog section of the hardware. This section is normally called RF front-end. Unlike the digital section, the analog section of wireless devices cannot easily become flexible. To be flexible without sacrificing its key performance measures such as selectivity, the RF front-end typically has to be physically large and complex. This is one of the main reasons why SDR and CR have not been widely implemented in mobile handsets.
Because every wireless application shares a signal medium, it is critical that each radio needs to receive only the desired signal and reject the others. The pre-selector filter, which is often placed at the input of receiver RF front-ends, plays a major role in extracting the desired signal from the overall signal that is captured from the air. However, the pre-selector filter is the part of the RF front-end which is least yielding for flexibility. Often, flexible pre-selector filters are either too big or very poorly selective.
The customary practice to address this problem has been to provide solutions, which make flexible pre-selector filters small and selective.